How should children aged 4 to 5 be secured in a vehicle?

The correct answer highlights the importance of using appropriate child-restraint systems based on the age and size of the child. For children aged 4 to 5, the safest practice is to secure them in the vehicle using a child-restraint system that is suitable for their weight and height. This can include either a forward-facing child safety seat or a booster seat when they outgrow the forward-facing seat but still need the added height to properly fit the vehicle's seat belt.

Using a child-restraint system is crucial for their safety, as it significantly decreases the risk of injury in the event of a collision. A seat belt alone does not provide the necessary support for younger children who may not be tall or heavy enough for it to fit properly over their body. Additionally, only using a booster seat or relying on a seat belt without a child safety seat overlooks the critical need for appropriate restraint methods as they transition in size and weight, making option A the best choice for ensuring their safety while traveling in a vehicle.